Mouse mammary tumor cells displayed switches in retrovirus expression - from type B to type C - during passage in culture. Though type B virus production was shut down, the late passage cultures continued to synthesize type B viral RNA and major structural proteins. However, some minor species of viral RNA may not be properly transcribed or processed in late passage cultures. Thus, the expression may be regulated at the level of processing of viral RNA or assembly of virus particle. Tumor promoter, TPA, stimulated the production of MMTV (type B) in early passage cultures that was in part related to increased synthesis of viral RNA. Interferon inhibited virus production without significant effect on viral RNA synthesis. Analysis of the sequence complexity of mRNA in human breast cancer cells in culture revealed three abundance classes. The total complexity of mRNA in these cells was higher than most other mammalian cells and it was not affected significantly by prolactin.